Magnetoresistance (MR) is the change in the resistivity of a material as a result of the application of a magnetic field. Ordinary magnetoresistance of non-magnetic metals and anisotropic magnetoresistance (AMR) of ferromagnetic metals and alloys have been known and observed for many years. The magnetoresistance defined as .DELTA.R/R is about 2% for an AMR material such as permalloy with magnetic fields of 5-10 Oe.
More recently, M. N. Babich et al., Phys. Rev. Lett. 61, 2472 (1988), reported giant magnetoresistance (GMR) in Fe/Cr multilayers in which there are alternate thin layers of Fe and Cr and the Fe layers are antiferromagnetically coupled. While such mutilayer systems have much larger magnetoresistance effects (45%), large fields of the order of 20 kOe are needed to produce the effect. GMR is also exhibited by spin-valves. Spin-valves are layer structures which are comprised of a "pinning" antiferromagnetic layer which serves to pin the magnetization of an adjacent ferromagnetic layer to a particular orientation. A second ferromagnetic, separated from the first by a non-magnetic conductor, can be rotated with relatively low fields, i.e., 10-20 Oe.
Colossal magnetorestriction (CMR) with .DELTA.R/R(H) of 100,000% or more has been observed (see S. Jin et al., Science 264, 413 (1994)) in lanthanum manganese films having perovskite structure with magnetic fields of 6 T.
CMR effects have been observed in Tl.sub.2 Mn.sub.2 O.sub.7 which has a pyrochlore structure (see Y. Shimakawa et al., Nature 379, 53 (1996), and in Tl.sub.2-x In.sub.x Mn.sub.2 O.sub.7. (see S-W. Cheong et al., Solid State Comm. 98, 163 (1996)). Greedan et al., Solid State Comm. 99, 399 (1996) report that the pyrochlore Sc.sub.2 Mn.sub.2 O.sub.7 is an insulator.
Magnetic read heads using AMR are in use commercially. They provide increased sensitivity over conventional thin film inductive heads. GMR and CMR provide the potential for even greater head sensitivity and CMR materials suitable for this use are needed. The present invention suits these needs and otherwise overcomes certain deficiencies of the prior art. Other objects and advantages will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.